Talc filled polypropylene and talc filled thermoplastic polyolefin materials are currently the principal materials that are used for exterior molded automotive trim. Reducing weight of future vehicles while increasing the amount of sustainable materials made from recycled and/or renewable materials is a problem that automobile manufacturers face. The primary ways to reduce the weight of plastic parts are to reduce the thickness of the part and to reduce the density of the material used to make the part.
The density of recycled thermoplastic materials is largely determined by the selection of the type and quantity of fillers. Talc is the primary filler used by most recyclers. Talc and other fillers are used to reinforce plastics to increase stiffness and strength. One example of a thermoplastic polyolefin formulation that uses recycled polypropylene is provided below:
recycled polypropylene55-65%polyolefin elastomer10-20%talc powder10-30%other additives 0-5% by weight.
Recycled polypropylene and thermoplastic polyolefin materials have higher melt flow rates when compared to virgin product. However, recycled polypropylene and thermoplastic polyolefin materials may still not have sufficiently high melt flow rates (35 to 50 g/10 min or higher, per ASTM D1238 or ISO 1133, at 230 C. and 2.16 kg load) and are difficult to process in injection molding machines that have thin walls (2.5 mm and thinner).
Recycled materials have melt flow rates that are determined by the composition of the materials that are being recycled. The melt flow rate of recycled thermoplastic materials is an average of the melt flow rate of the ingredients. The melt flow rate of the recycled material may limit minimum part thickness in injection molding processes. Increasing melt flow rates of injection molded materials is a key to molding thin walled parts. Part performance and appearance must be maintained as the wall thickness of parts is reduced. Material stiffness must be significantly increased to 2,000 MPa flexural modulus or greater to maintain performance in parts having thinner walls.
Talc and other fillers are primarily used to reinforce plastics and increase stiffness and strength. Talc also stabilizes plastics by reducing the Coefficient of Linear Thermal Expansion (hereinafter “CLTE”). Talc acts as a nucleating agent enhancing material solidification in the injection molding process. Talc increases the speed of nucleation (solidification) of the material as it is cooled in a mold. Conventional fillers like talc tend to adversely increase part density, reduce material ductility and degrade surface quality. Talc has a density of about 2.6 g/cm3 (ASTM D792-08) and is the primary filler used in automotive plastics. Automotive plastics typically use between 10% and 30% by wt. talc as filler.
Other reinforcements such as glass fiber, carbon fiber, and nano-composites may be relatively heavy or abrasive. Carbon fiber, nano-composites and other reinforcements are used to stiffen polypropylene and thermoplastic polyolefin materials to a greater extent than is feasible with talc. Abrasive fillers cause tool wear over time that may degrade part appearance. The abrasiveness of the material wears out steel tools making the filler unsuitable for high volume automotive trim parts that have a class “A” surface. In addition, carbon fiber and nano-composites are in limited supply, expensive and difficult to mold.
The fillers in filled polypropylene and talc filled thermoplastic polyolefin materials typically range from 10 to 40% by weight. The density of unfilled polypropylene density is approximately 0.9 g/cm3. For example, 20% talc filled polypropylene, polyethylene, or polyolefin materials have a density of about 1.04. In another example, 40% talc filled polypropylene, polyethylene, or polyolefin materials have a density of 1.22. The relatively high density of talc offsets any weight savings that may be expected as a result of reducing part wall thickness.
The above problems and challenges are addressed by this disclosure as summarized below.